One of the known ways of foaming polymer compositions is to use polymeric hollow microbeads, known as microballoons. Microballoons have their own polymer shell, inside which is a liquid which by means of a treatment step—generally by supply of heat or by generation of heat—is caused to expand into the gas phase with an accompanying softening of the polymer shell to an extent such that it is able to increase the volume it encloses. Polymers foamed with microballoons have been known for a long time and described for a long time, including in the form of permanently tacky compositions. The words “permanently tacky composition” mean the same as “self-adhesive composition” or “pressure-sensitive adhesive”. The prior art includes DE 21 05 877 C, EP 1 102 809 A1 and DE 195 31 631 A1.
The known types of polymers foamed using microballoons, more particularly self-adhesive compositions, have not only the cavities generated by microballoons and often called “caverns”) but also cavities which are not surrounded by their own polymer shell. The presence of caverns not surrounded by their own polymer shell has the effect, to the knowledge of the inventors, of promoting the formation of open-celled foams.
To the knowledge of the inventors, however, it would be better to have foamed polymers with caverns produced only by microballoons, since these polymers achieve a more uniform size distribution of the hollow cells, and a higher cohesion, without disadvantages in terms of adhesion. In a few applications, moreover, the higher permeation resistance to gases and liquids that is achievable by the closed-celled nature of the polymers is desirable. Moreover, such caverns without their own polymer shell tend to collapse under the influence of pressure and/or heat, and/or combine to form large bubbles.
Materials which have been foamed—as specified in publication DE 21 05 877 C, for example—not by means of microballoons, but instead in another way, are more susceptible, by contrast, to irreversible collapse under pressure and temperature. They also have a lower cohesive strength.
Polymer compositions foamed by means of microballoons exhibit the same disadvantages if too many microballoons collapse in the process. According to EP 1 102 809 A1, skilled workers expect such collapse if the microballoons expand at such an early point in time that thereafter they are exposed to considerable stresses, whether as a result of shearing or as a result of heat-induced expansion pressure. The softness of the membrane, as a consequence of the high temperatures generally prevailing in the process, and also its thinness, as a result of the stretching that is associated with the expansion, cause the membrane to tear easily, thereby releasing the blowing gas in the microballoons as an unstabilized gas bubble in the polymer foam. Consequently, premature expansion is to be suppressed.
Known from DE 195 31 631 A1 is a process in which, admittedly, the late disposition of the expansion step appears to rule out the destructive tearing of the microballoon membranes, but in which, when the granule-like microballoons and other adjuvants are incorporated, entrains air into the polymer matrix and hence once again, deleteriously, generates caverns of the kind whose boundary area is not sealed and stabilized by its own polymer shell.
It is an object of the invention to provide a foamed polymer composition—preferably a self-adhesive composition—which is free from cavities of the kind not generated by microballoons. Besides expandable microballoons, non-expandable microballoons, if any at all, hollow glass beads, for example, are to be used for achieving foaming, but free gas bubbles are to be suppressed.
A cavern originating from an expandable microballoon has a membrane-like shell comprising a polymer or polymer mixture, and which—in spite of every stretchability achieved by heating—has a higher elasticity than the surrounding polymer composition matrix at the cooler application temperatures of the completed product of the invention.